The present invention applies generally to a filter apparatus for removing suspended solids from a fluid stream that is passed through filter material. More particularly, the present invention relates to methods and apparatus for cleaning the filter material.
The invention claimed relates generally to the cleaning of rotating filter media commonly employed in the treatment of fluids such as water, wastewater and industrial process streams. Such filters often employ textile cloth membranes of cellulose base material, other natural fibers or synthetic fibers woven or napped into a tight fabric or matting. The cloth filter material is stretched over large drums or multiple disk-type frames. For non-limiting examples, see U.S. Pat. Nos. 4,090,965 and 4,639,315.
Typically, the filter media is placed in the flow path of a fluid stream containing suspended solid particles which are to be removed by the filtering process. The solid particles larger than the openings in the filter media are retained on the upstream, or influent, side of the filter media while the remaining flow (the filter effluent) passes through. Over time, these solids build up on the influent side of the filter media and impede the rate of filter effluent that passes through, thus necessitating a cleaning of the filter to remove the solids build-up.
Two common cleaning methods known in the art are forward pressure washing and reverse flow backwashing. Forward high pressure washing is required when normal low pressure backwashing cannot assure ultimate media cleanliness and when filtration cycles progressively shorten. A pressure washing cycle will reconstitute media cleanliness and reestablish acceptable media headloss. Typically, several backwash events will occur between pressure washing cycles. Each pressure wash operation may require the filter to stop processing influent, whereas the backwashing operation typically does not. Pressure washing is a process that applies a pressurized water spray from a series of nozzles evenly displaced along a stationary spray arm positioned to span a generally radial distance across the filter media. Other filter models may operate with fixed filter panels and moving pressure spray arms and backwash headers.
In the pressure spray applications, the pressurized spray delivered by the nozzles dislodges the accumulated solids on the filter media in part by overcoming the adhesive force of the solids against the influent side and dislodging them from the filter media or in part by driving embedded solids particles through the filter media into an effluent channel. Therefore, the effectiveness of the cleaning process includes the application of a sufficient washing flow volume and a sufficient spray pressure. Ideally the application of the wash flow and spray pressure would be evenly distributed across the filter media, but inherent limitations in the current mechanical design of spray arms and nozzle configurations and their angular orientation prevent known systems from functioning in this optimized condition. Moreover, it is desirable to minimize the length and frequency of filter cleaning cycles. Consequently, a cleaning process that cleans unevenly or fails to effectively remove the embedded solids will require more frequent cleaning and will produce less filter effluent.
It is a known problem with current methods of rotating disk filter media cleaning that the area of the rotating filter media nearest the axis of rotation is cleaned more thoroughly than the more radially distant areas. This is primarily due to the substantially higher relationship of applied wash flow per unit of filter area at the inner portion of the rotating filter media in the prior art. It is also a known problem that the filter areas which pass directly under the center portion of a pressurized spray nozzle are cleaned more thoroughly than the filter areas that pass under the space between nozzle centers, due to significant variations in the contact angle of individual jet streams emitted by the spray nozzles. The present invention provides improved cleaning methods and structures that overcome these and other limitations of current pressurized spray cleaning techniques.
A significant advantage of the present invention over prior art methods and devices for cleaning filter material is that the method and apparatus of the invention maintains the filter material in a uniformly clean, and thus, more effective filtering condition for longer periods of filter operation. This advantage results because the wash flow rate and hydraulic energy per unit of filter area are applied more evenly across the filter media, compared to prior art filter cleaning devices and methods. The invention significantly reduces the number of wash cycles required by the filter and, thus, highly efficient filtration is achieved.
It is a feature and an advantage of the present invention to provide a method and apparatus for cleaning filter media that does not subject the filter media to harsh operating conditions, thereby extending the operating life of the filter material.
It is also a feature and an advantage of the present invention that the nozzles on the stationary spray arm rotate the major axis of the spray fan contact area into an angular relationship with the axis of the spray arm to create a spray overlap condition.
It is also a feature and advantage of the present invention that the method and apparatus described can be positioned to apply a pressurized spray to either the influent side or the effluent side of the filter media.
It is also a feature and advantage of the present invention to improve media cleaning performance and increase filtered water production and reduce required spray water pressure, energy, washwater consumption, waste washwater volume and overall cost.
Definition of Terms
The following terms are used in the claims of the patent and are intended to have their broadest meaning consistent with the requirements of law:
filter mediaxe2x80x94 any permeable material, including but not limited to natural or synthetic fiber based, granular or membrane compositions;
nozzlexe2x80x94 any device or orifice type opening which disperses wash water from a spray arm;
concentric bandxe2x80x94 a generally circular geometric strip having a width of a radial distance;
radial distancexe2x80x94 a length measured along a straight line which intersects the center of a rotating filter element; and
spray armxe2x80x94 any conduit for transmitting pressurized wash water to a spray nozzle or a plurality of spray nozzles.
Where alternative meanings are possible, the broadest meaning is intended. All words in the claims are intended to be used in the normal, customary usage of grammar and the English language.